chaps need to be reasonably clean.
The first problem with all the chaps is none are a guarantee when facing a modern saw.
Second is that you have to properly maintain them. Your concern on the oil may well be justified. If the protective fabric, inside the chaps is bonded by bar oil/grease/dirt, that can make it easier to cut your Kevlar or ballistic nylon etc. This is counter to what you need; strands of fabric that are not cut but ones that get pulled out of the chaps and into the sprocket area.
If your chaps are newer the outer fabric, (that doesn't provide any of the protection for you - but does protect the important stuff inside), should have kept the oil out. Don't worry about a chemical reaction with oil weakening the fabric layers. {Unless you use a substance like chlorine to clean them, chlorine is very hard on Kevlar.)
I'm going to make a guess and say that the manufacturer will strongly recommend cleaning and have a specific method for just this instance.
It’s been several years for me, but most all of us have done the gas or oil cap thing. Bet the chaps’ manufacturer has seen this a few hundred times before.
Quick caveat in only a slightly miss-directed thread. We had a Hotshot two summers ago dump mixed gas all over a leg. He didn't think much of it and just gassed up, put the lid on this time, went back to work. Had a loose plug wire that sparked, went to a burn ward. Not major burns but did get a skin graft.